The present invention relates to the formation of ion-implanted emitters in transistor integrated circuits and more particularly to a high current implantation process through a screen oxide layer which eliminates damages and contamination caused by high current implantation. The process avoids nitride overhang so that passivation of the emitter/base junction is achieved.
Ion implantation through a screen oxide layer into a semiconductor substrate to form impurity regions is described, for example, in U.S. Pat. No. 3,945,856. The oxide layer has a thickness of from 100 to 1,000 A and contaminating ions from the ion implantation equipment are trapped in the upper portion of the oxide layer. The upper portion is generally damaged. This portion of the oxide layer is etched away prior to the high heat drive-in step so that the contaminants are not diffused into the substrate.
Ion implantation of emitters into a diffused base has the advantage over a diffusion process in that the impurity dosage can be closely controlled to within 1-2% obtaining tight parametric distributions, and performance enhancement, so that device yield should increase. A problem arises, however, with proper passivation of the emitter/base junction because of the nitride overhang and directionality of the implant which confines the implanted area to the opening in the mask. Therefore, if the passivating layer is undercut beneath the mask opening, the layer will not properly overlap the implanted emitter/base junction at the surface of the substrate and device yields are reduced. This undercutting occurs when etching through composite layers of silicon nitride and silicon oxide to open the emitter area where a difference in the thickness of the oxide layer between collector and emitter requires an overetch of the emitter opening in order to remove a thicker oxide layer portion over, for example, a collector or Schottky barrier diode contact region.
The problem of undercutting has been previously addressed in the IBM Technical Disclosure Bulletin article, "Fabricating Ion Implanted Regions in Semiconductors" by J. Doulin et al. Vol. 19, No. 9, February 1977, pages 3407-3408 and "Implanted Emitter Process For Semiconductor Transistor" by C. Barile et al., Vol. 18, No. 7, December 1975, pages 2177-2178. In the Doulin et al. article undercutting is "minimized" by stripping the oxide layer over the collector and then regrowing the oxide. This still results in a differential in oxide thickness over the emitter and collector contact regions so that some overetch of the emitter region is still needed to open the collector contact regions. In the Barile et al. article, undercutting is avoided by using an etchant for the oxide which also attacks the overlying nitride. This eliminates nitride overhang but at the expense of enlarging the emitter and contact openings. Alternatively, reactive ion etching is employed to etch the oxide.
However, because of the non-selectivity of this type of etch, where different oxide thicknesses must be etched, the overetch of the emitter opening needed to open the collector contact can result in attack of the semiconductor surface by the reactive ion etch process leading to severe pipe problems.